Strobe devices require the application of a relatively high voltage across a flash tube in order to produce a gaseous discharge in the tube. In known devices, this high voltage is achieved by using a charge pump to transfer energy to an internal capacitor from an external energy source. The external source typically can be nominally 12 volts or 24 volts.
The capacitor is coupled in parallel with the flash tube and provides the energy for the flash. The amount of light from the flash tube is directly proportional to the energy stored in the capacitor that is discharged into the flash tube.
A single capacitor can be charged to various voltages in order to provide a multi-candela (multi-intensity) unit. However, there are limitations on the range of candela (intensity) that can be reliably achieved. One problem is that to flash the tube requires that the voltage across it be greater than a predetermined threshold amount (e.g. 180 volts) for reliable operation.
Present designs for multi-candela strobes include a range of 15 candela to 100 candela. To achieve such outputs, the capacitor needs to be charged to 240 volts for the 100 candela, but will only need to be charged to 120 volts for the 15 candela output. The 120 volts is, however, below the exemplary 180 volts needed for reliable operation.
In order to overcome this low voltage problem, known designs incorporate a voltage booster circuit to increase the voltage across the flash tube. One type of a voltage booster circuit is a voltage doubler circuit. One known voltage doubler design is disclosed in a “flashtubes” EG&G Heimann Optoelectronics Catalog, pg. 7, 1991. This document discloses a voltage doubler circuit to be used with a flash tube.
A prior art strobe unit with a known doubler is illustrated in FIG. 1. A capacitor C3 stores the energy that is going to determine the candela of the flash. It is coupled across a series combination of a flash tube LP1 and a diode D13.
Capacitor C13 is the doubler capacitor. It is charged through resistor R15 to the same voltage Vc as capacitor C3 is charged. The polarities of the voltages on the capacitors C3 and C13 are the same. Capacitor C4 is used for the trigger function and is charged to the same voltage and polarity as is capacitor C13. Hence, the trigger voltage, across capacitor C4 tracks the value of Vc. As Vc varies, based on desired candela output, so does the trigger voltage. Thus, the trigger voltage may be excessive at high candela outputs. Additionally, at low candela settings, the trigger voltage may not be high enough to produce reliable ignition of the flash tube LP1.
When the unit is triggered, by a signal from the trigger circuit, SCR Q8 will conduct and pull node A low. This causes C4 to discharge through Q8 and the primary winding of TR2, the trigger transformer.
Until the flash tube is triggered by the voltage out of the secondary winding of TR2, C13 and C3 cannot discharge. However, the voltage across the flash tube at this time is double the voltage VC of C3 (far exceeding the minimum required voltage). In FIG. 1, the voltage across capacitor C4 is the same as the voltage stored in the capacitor C3. The turns ratio of transformer TR2 is designed to provide 6 KV output when the capacitor C3 has been charged up to about 220 volts. As noted above, at low candela settings, the trigger voltage may be too low to be sure that the tube will be triggered. At high candela settings, the voltage can become so high that the transformer may start to arc and to fail.
When the tube flashes, it first discharges capacitor C13, then capacitor C3. The energy stored in capacitor C3 provides the preselected candela output from tube LP1.
While known devices provide a selectable candela output, the use of a voltage doubler does have some disadvantages. At high output intensities, the voltage across the tube LP1 is substantially equal to 2 VC which can be quite high. This high voltage requires the use of components rated therefore. In addition, in compact units with the circuitry implemented on a printed circuit board, arcing is a potential problem. Further, it would be preferable if the trigger voltage was not dependent on candela setting since the voltage Vc can vary widely as a function thereof.
There thus continues to be a need for multi-candela strobe units which provide reliable, triggerable light of a selected intensity. Preferably such reliability could be achieved in compact, high density packaging, without the necessity of high voltage components. It would also be preferable if operational reliability could be achieved while simultaneously eliminating arcing during normal operation.